How to Insert Into A List With Slicing In Python

Used to solve Leet Code’s Duplicate Zeroes Puzzle

Pavol Kutaj
2 min readSep 14, 2021

The aim of this page📝 is to utilize Python slicing to insert a data structure into a list at a given index. Slicing indexes operate as a half-open range — the first item is included, the last is not:

>>> l = [1,2,3,4]
>>> l[0:1]
[1]
  • to insert at a given point, start pointing to the item you want the new structure to start with. then, use the same index value after
  • e.g. [0:0] to insert a sublist of a random length in the very beginning of a list
  • the identical values in ([0:0]) are not specifying the span, they only point to the point of entry
  • in a stupid example below I want to add 666 at the beginning of the list [1,2,3,4]
>>> l = [1,2,3,4]
>>> l[0:0] = [6,6,6,]
>>> l
[6, 6, 6, 1, 2, 3, 4]
>>>

EXAMPLE: LEET CODE’S DUPLICATE ZEROS PUZZLE

  • I’ve used the technique to easily solve https://leetcode.com/problems/duplicate-zeros/ which seems rather challenging in an official solution
  • It goes like this: Given a fixed-length integer array arr, duplicate each occurrence of zero, shifting the remaining elements to the right.
  • Note that elements beyond the length of the original array are not written.
  • Do the above modifications to the input array in place and do not return anything.
# EXAMPLE
Input: arr = [1,0,2,3,0,4,5,0]
Output: [1,0,0,2,3,0,0,4]
Explanation: After calling your function, the input array is modified to: [1,0,0,2,3,0,0,4]
  • I only insert at the point where the zero is found with inserting with slices
  • Then, I remove the last item by with pop()
  • Finally, I move the i pointer by two to reflect the insertion as we're mutating a collection that's also being iterated upon
# SOLUTION
def duplicateZeros(self, arr: List[int]) -> None:
i = 0
while i < len(arr):
if arr[i] == 0:
arr[i:i] = [0]
arr.pop()
i += 2
else:
i += 1

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Pavol Kutaj

Today I Learnt | Infrastructure Support Engineer at snowplow.io with a passion for cloud infrastructure/terraform/python/docs. More at https://pavol.kutaj.com